Our recent results provide evidence that one of the photoproducts of the rhodopsin-like pigment found in the distal cells in the eye of Pecten interacts with the membrane conductance to depolarize the membrane. Blue light alone (wavelength 500nm) is associated with membrane hyperpolarization which can last beyond the time of illumination. Red light alone (wave-length 500 nm) produces a similar hyperpolarization at onset of the stimulus, but membrane voltage returns rapidly to the dark level after the flash. Red light given during the sustained hyperpolarization which follows blue light results in immediate depolarization of membrane voltage to the pre-flash dark level. Potential changes elicited by intense light flashes include an early receptor potential (ERP) component. Membrane hyperpolarization voltage to pre-flash dark levels is correlated with a depolarizing ERP. Our evidence suggests the existence of two photo-pigments which are thermally stable and interconvertible by light. Photo absorption by one (max 500 nm) is the initiatory event preceding membrane hyperpolarization. Photon absorption by the other (max 500 nm) triggers membrane depolarization. BIBLIOGRAPHIC REFERENCES: McReynolds, J.S. and A.L.F. Gorman (1975). Hyperpolarizing photoreceptors in the eye of a primitive chordate, Salpa democratica. Vision Res., 15: 1181-1186. Cornwall, M.C. and A.L.F. Gorman (1975). Color dependent potential changes of opposite polarity in single visual receptors of Pecten irradians. Biophysical J., abstract (in press).